1. Field of the Invention (Technical Field)
The present invention relates to the field of communication over microprocessor buses. In particular, the present invention relates to minimizing radio frequency radiation from a microprocessor bus resulting from digital communication over the bus.
2. Background Art
There are a variety of digital systems that rely on microprocessors for system control. Microprocessors control operations including user interaction—such as keypad decoding, modem operations, radio operations—if applicable, control of the display of data to a user, control of storage devices (memory) and control of other “peripheral” devices. “Peripheral” devices include, for example, displays, printers, scanners, digital cameras and memories as well as a variety of other devices. The microprocessor communicates with peripherals over a microprocessor bus.
Typically the microprocessor can operate at much higher speeds than the peripherals, and in particular at higher speeds than many low-cost memories. In order for a microprocessor to communicate effectively with a variety of peripherals having unique addresses and differing communication response periods, “wait states” are used so that the microprocessor does not send out signals to the peripheral at a speed that the peripheral cannot accommodate. The number of wait states used when communicating with a particular peripheral corresponds to the peripheral address. Wait states are additional machine cycles that provide additional access time for external memories or peripherals having slower response times. Such wait states give the peripherals time to receive and respond to the communication signal from the microprocessor, before the next signal is sent from the microprocessor. For example, a printer may require more wait states because the cables connecting it to the microprocessor are physically longer, resulting in a time delay between the time that the signal is sent and the time that it is received. A printer may also require more wait states because it operates at a slower rate and may be slower to respond and decode signals received. In contrast, a liquid crystal display (LCD) typically requires fewer wait states as it is often located more proximate to the microprocessor and has a fast response time.
Memory devices, as well as many other peripherals, typically are set to operate with the fastest achievable wait state because access is needed on a frequent basis. However, peripherals will operate effectively with a higher number of wait states than the minimum required for the response time, although implementation of a higher number of wait states will slow communication.
In addition to the need for control of the timing of communication over the microprocessor bus with wait states, other difficulties can arise from communication over the microprocessor bus. One such difficulty is that unwanted radio frequency interference (rfi) can radiate from the bus. Electromagnetic radiation is generated whenever electrical current travels along a conductor. This radiation can propagate to other devices in proximity to the conductor.
In the case of a digital pulse, the rate of change of the pulse voltage determines the frequency spectrum available to radiate. The frequency spectrum of a digital pulse traveling along a conductor, or microprocessor bus, can extend into the radio frequency (rf) range. For example, when a fast digital pulse travels along the microprocessor in a digital system, rf radiation is typically produced in the vicinity of the bus. The operating frequency of the microprocessor bus is the source of the fundamental frequency for radiation from the bus, and generally the strongest effects of rf interference are due to harmonics of this frequency. Further, a typical digital pulse transmitted along the microprocessor bus has a rise time from 0 volts to 3 volts in the range of 10 nsec. A rise time of this speed produces high frequencies, in the range of 100 MHz, which are more readily radiated from the bus because the relationship between the wavelength of these frequencies is closely matched to the physical size and geometry of the bus; the efficiency of radiation from a conductor at specific frequencies is dependent upon the geometry of the conductor.
The orientation of the bus to another device with which the bus may cause rfi, such as an antenna, effects the degree of interference experienced by the antenna. If the bus is both orthogonal to the antenna and minimal in length, then the antenna will have optimal interference rejection capability. However, if the bus is not orthogonal to the antenna and has a length of more than 1/10 of the wavelength (λ) of the potentially radiated frequency, then the probability of rfi interfering with the receiving capability of the antenna increases.
Additional rf radiation from the bus can occur if it is not terminated correctly, because a reflected pulse can propagate back in the opposite direction from the terminal end of the bus.
Other factors contributing to rfi from a conductor, or bus, include the length of the conductor, and its proximity to a quality ground. A longer signal path typically results in increased total rf radiation, depending upon the wavelength of the conducted signal and the type of conductor.
Therefore, undesired rfi is emitted from the microprocessor bus when signals are transmitted between the microprocessor and peripheral. This interference can propagate to other electronic components in the vicinity of the microprocessor bus, via electromagnetic radiation or conduction, generating noise and interfering with their efficient and reliable operation.
One way to mitigate rfi from the microprocessor bus is to shorten the length of the bus. The disadvantages of shortening the length of a bus are obvious; the signal path length to peripherals is constrained by the length of the bus. This solution requires designing the landscape of the system components within the confines of the available signal path lengths.
Another way of mitigating rfi from the microprocessor bus is to provide Faraday cage shielding around the bus to confine the electromagnetic radiation, thereby inhibiting the radiation and conduction of the undesired rfi. The Faraday cage is typically placed around the microprocessor and peripheral device or devices. A disadvantage of Faraday shielding is that it requires the shield be made of metal, or at least metallized-plastic, which can be prohibitively expensive.
A third method used to mitigate rfi is to implement buffering schemes, or gating, in the path of the microprocessor bus. A buffer is used to prevent the passage of a signal beyond the location of the buffer. The buffer essentially operates as a switch allowing continuity of the signal path at those times when communication must occur between the microprocessor and the peripheral device. If the buffer is “off” no signals can pass the buffer, and radiation is not generated from the bus beyond that point. If the buffer is “on”, signals are allowed to pass down the bus. Typically, such buffers are implemented with a simple digital logic gate. For example, to buffer a particular peripheral, a two-input AND gate is placed in the signal path between the microprocessor and peripheral. The data signal to the peripheral is fed into one input of the AND gate, and the enable for that peripheral is fed into the other input. The output from the gate corresponds to the data input, so long as the enable input is a digital high, equivalent to the “on” state.
Buffering is simple to implement and limits the occurrence of pulses causing rf radiation, but it does not eliminate the radiation entirely. Once a data pulse passes through the buffer, in the “on” state, the pulse generates rfi in that portion of the path beyond the buffer.
Digital systems used for wireless communication, including mobile cellular and satellite telephones, pagers, personal digital assistants (PDAs), and the like (hereafter “mobiles”) particularly suffer from rfi from the microprocessor bus. Mobiles typically comprise a rigid housing enclosing a printed circuit board, electronic and electro-acoustic components, a portable power supply such as a battery, and an associated microprocessor for control of the device. Mobiles communicate through a variety of means, primarily through antennas that transmit and receive radio frequency (rf) signals, but also through infrared (IR) emitters and receivers, or cable connections to input/output ports of computers and other mobiles. The user interfaces with the circuitry and microprocessor of the mobile through a keypad located on the front outer surface of the housing. Keys on the keypad are pressed by the user to temporarily close an internal switch and send a signal to the microprocessor where an appropriate routine processes the input and operates the mobile. A display on the housing provides a readout of data input by the user and data received by the mobile, access to spatially navigated menu trees, and graphical user interfaces (GUIs). Radio frequency interference from the mobile microprocessor can interfere with the reliable operation of the mobile, in particular with the reliable operation of mobile communications over radio frequencies.
It is common for rfi from the microprocessor bus of a mobile, such as a cellular telephone, to cause an apparent reduction in sensitivity of the rf receiver of the mobile in the range of 10 dB. This reduction results from the combined effect of increased phase noise contribution, unwanted mixing of spurious tones, and receiver compression which distorts the received signal. The rf signal radiated from the microprocessor bus is a broadband function with a sinc (sin(x)/x) function overlay at the clock frequency of the bus at the access rate. Thus, harmonics of the access frequency as well as a general increase in the apparent noise floor of the received rf signal can be attributed to microprocessor bus activity.
End user groups require that digital radio transceivers encode and decode communication signals within stated bit error rates (BERs) so that communication reliability is maintained in the face of interference resulting from operation of the mobile itself. Radio frequency interference can also lead to difficulties with approval of mobile operations, in particular, because spurious radiation of rf from the mobile microprocessor bus and/or from other sources can interfere with the ability of another mobile in the band of interest to properly decode incoming rf communication signals.
A simple solution is needed to mitigate rfi from microprocessor buses due to digital pulses travelling on the bus. Such a solution would ideally be inexpensive, would not compromise the physical length of the bus, and would not interfere with other components in the system used to control communication flow.